Some components made of fibre-reinforced resin matrix composite materials periodically require repair if the surface of the composite material has become damaged during use. Some of these components are installed as part of structures or machines and/or are so large, for example comprise wind turbine blades which may exceed 50 meters in length, so that it is technically and economically expedient to repair them in situ in the field.
It is known to use a wet lamination repair process in which dry fibrous layers and a liquid resin are laid up over the damaged surface and the liquid resin is then cured to repair a structure in situ in the field. In order to ensure resin compatibility, the type of resin selected for a repair usually is the same generic family used to manufacture the original article and most commonly would be polyester, vinyl ester, or epoxy resin. Polyester, vinyl ester repairs are usually done above 15° C. as these resins are difficult to cure and adhere below this temperature and can exhibit permanent undercure even when subsequently heated if gelled at a low temperature. Epoxy resins can be formulated to adhere to polyester, vinyl ester as well as epoxy substrates and are often the material of choice. However such conventional curing wet-laminating epoxy resin are prone to “bi-product” at low ambient temperatures and at high ambient humidity, such a bi-product phenomenon resulting from the amine curing agents tending to react with atmospheric carbon dioxide and moisture to form an ammonium carbamate layer causing poor subsequent over laminating adhesion. A further consequence is that the amine compounds intended to react with the epoxy compounds are consumed in the reaction with moisture and carbon dioxide. As a consequence, not all the epoxide groups are reacted, leaving under cured resin resulting in low mechanical and thermal performance.
A further problem at low temperatures is providing a balance of pot life, when the resin is mixed, and when the resin is spread out to form a thinner layer on the substrate. To cure at low temperatures, reactive compounds are needed to cure the resin as a thin layer which makes it very difficult to mix the resin due to very short pot-lives.
To repair a part where access is difficult or limited (for example for making an in situ repair of a wind turbine blade), it is common practice to first inspect the damage and cut the required fabric (e.g. of fibre glass) to shape. The fabric is placed on a backer and the liquid resin is then used to impregnate the fibres, thereby forming a “wet preg”. These wet preg preforms are difficult to handle because of the presence of liquid resin. If the fibre glass fabric is first cut to shape, the act of wetting out and handling the fabric distorts the shape and it no longer is a good fit to the original defect shape and the desired fibre alignment can be lost. The liquid resin tends to drip and cover the non repair areas requiring substantial post repair clean up. The liquid resin tends to transfer onto the operator's protective gloves, and then easily onto tools, clothing, and the non repair areas. When access is difficult, such as repairing a wind turbine blade from a rope line or platform, often in windy conditions, the handling problems are even more prominent.
Therefore such a wet lamination repair process cannot reliably be used under certain environmental conditions in the field. Also, wet laminating is messy and time consuming in the field.
It is also known to use conventional prepregs in a repair process in the field. However, such prepregs require initial vacuum bagging, to evacuate air from between the prepreg and the substrate and from between adjacent prepreg plies, and subsequent heating, to provide low enough viscosity and consolidation pressure to ensure sufficient wetting between the prepreg plies and substrate, full fibre wet out if the prepreg was not already fully impregnated, and enough time at temperature to cure the prepreg resin. This is difficult and time consuming to do reliably, particularly to ensure that the prepreg laminate is fully and uniformly heated to ensure wet out and resin curing throughout the laminate. When using both conventional thermal cured and UV curable prepregs to repair the leading edge of a wind turbine blade, it has been found that applying the pre-preg directly to the damaged surface under ambient temperature and without additional heat results in no or only very poor adhesion between the prepreg and the substrate as the prepreg is not able to easily wet the substrate surface. A heat rolling step can be used to adhere a conventional prepreg, but this process extends the application time and still only provides poor adhesion. In the field it is difficult to guarantee that the required temperature between the prepreg and the substrate has been achieved uniformly over the surface area of the repair. Additionally, even with heating it is difficult to cause the prepreg resin to flow in order to reduce any resin high spots or to fill with resin any low spots in the substrate. At the edges of the prepreg ply, where there is a height difference between adjacent prepreg plies, drop offs can occur leading to more voids in the repair. Furthermore, if resin heating is required it is necessary to provide a common heater mat for different sized and shaped damaged areas.
U.S. Pat. No. 5,732,743 discloses a method of sealing pipes, for example to join or repair the pipes, using a prepreg. However, the prepreg and method disclosed do not provide a durable repair, would be unsuitable for repairing large areas, such as on a wind turbine blade, and would be difficult to apply in the field when exposed to ambient environmental conditions. U.S. Pat. No. 5,166,007 discloses a patch or repair assembly, including a prepreg, for repairing vehicles. U.S. Pat. No. 5,554,666 discloses a photocurable putty or molding. US-A-2004/0067335 discloses a method of repairing damaged concrete structures using wet-laminating of fibre-reinforced polymer composites. EP-A-0025359 discloses photocurable prepregs. EP-A-0922727 discloses photocurable prepregs for waterproofing.
None of these prior art documents provides a fibre/resin combination or method which enables the reliable and speedy production of a durable repair of a substrate, in particular over a large area, such as on a wind turbine blade, which can be applied in the field when exposed to a variety of ambient environmental conditions.